Definition and causes-acute mesenteric ischemia

• Reduced or absent blood flow to small intestine, typically caused by arterial (ie, SMA or its branches) occlusion or transient hypoperfusion or less often by venous occlusion

• Arterial embolism (~40–50%): embolic occlusion to SMA (has narrow take-off angle), often in setting of AF, valvular disease incl. endocarditis, atherosclerotic plaque in aorta

• SMA thrombosis (~20–30%): typically due to atherosclerosis at origin of SMA; other risk factors incl. vascular injury from abd trauma, infxn, or mesenteric dissections/aneurysms

• Nonocclusive mesenteric ischemia (~10%): transient intestinal hypoperfusion due to ↓ CO, athero, sepsis, drugs that ↓ gut perfusion (pressors, cocaine, amphetamines)

• Mesenteric venous thrombosis (MVT, ~5%): a/w hypercoag. states, portal hypertension, IBD, malignancy, inflammation (pancreatitis, peritonitis), pregnancy, trauma, surgery

• Focal segmental ischemia of small bowel (<5%): vascular occlusion to small segments of small bowel (vasculitis, atheromatous emboli, strangulated hernias, XRT)